Question

I am trying to read a file using fstream. Then when I reach the end I append the file and write some staff
file

abc

I wrote sample code

#include<iostream>
#include<fstream>
#include<queue>

using namespace std;
int main(int argc, char **argv){
    fstream *binf;
    binf=new fstream("t.txt", ios::out|ios::in|ios::app);
    cout<<"EOF"<<EOF<<endl;
    while(true){
        cout<<"before peek"<<endl;
        cout<<"binf->tellg:"<<binf->tellg()<<endl;
        cout<<"binf->tellp:"<<binf->tellp()<<endl;
        cout<<"binf->peek()::int=("<<(int)binf->peek()<<")::char=("<<(char)binf->peek()<<")"<<endl;
        cout<<"after peek"<<endl;
        cout<<"binf->tellg:"<<binf->tellg()<<endl;
        cout<<"binf->tellp:"<<binf->tellp()<<endl;
        char c;
        if(binf->peek()==EOF){
            cout<<"file is not good"<<endl;
            break;
        }
        binf->get(c);
    }
    cout<<"binf->tellg:"<<binf->tellg()<<endl;
    cout<<"binf->tellp:"<<binf->tellp()<<endl;
    binf->seekg(3);
    binf->seekp(3);
    cout<<"binf->tellg:"<<binf->tellg()<<endl;
    cout<<"binf->tellp:"<<binf->tellp()<<endl;
    binf->put('H');
    cout<<"binf->tellg:"<<binf->tellg()<<endl;
    cout<<"binf->tellp:"<<binf->tellp()<<endl;
    binf->seekg(4);
    char c;
    binf->get(c);
    cout<<"c:"<<c<<endl;
    binf->clear();
    binf->close();
    delete binf;
    return 0;
}

This is the output I get from running this code

EOF-1
before peek
binf->tellg:0
binf->tellp:0
binf->peek()::int=(97)::char=(a)
after peek
binf->tellg:0
binf->tellp:0
before peek
binf->tellg:1
binf->tellp:1
binf->peek()::int=(98)::char=(b)
after peek
binf->tellg:1
binf->tellp:1
before peek
binf->tellg:2
binf->tellp:2
binf->peek()::int=(99)::char=(c)
after peek
binf->tellg:2
binf->tellp:2
before peek
binf->tellg:3
binf->tellp:3
binf->peek()::int=(10)::char=(
)
after peek
binf->tellg:3
binf->tellp:3
before peek
binf->tellg:4
binf->tellp:4
binf->peek()::int=(-1)::char=(ÿ)
after peek
binf->tellg:-1
binf->tellp:-1
file is not good
binf->tellg:-1
binf->tellp:-1
binf->tellg:-1
binf->tellp:-1
binf->tellg:-1
binf->tellp:-1
c:

All I am trying to do is to fix the file and write at the end
however, whenever I see the EOF at the end of file the file is no longer good to use
even with peek()

Answer 1

When you hit the end of file you put your fstream into an error state. When it’s in an error state nothing will work until you clear the error state. So you need this

   if(binf->peek()==EOF){
        cout<<"file is not good"<<endl;
        binf->clear(); // clear the error state
        break;
   }

You do not need to clear the fstream just before you close it, that does nothing.

BTW good debugging technique, but you’ll find this easier if you learn to use a proper debugger.